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Drat. That does look like the right tool for the job, although 28.6mm is a standard seat clamp size, so I don't see why you couldn't use one of those if you were to file the lip off. It would certainly be cheaper.
It's interesting in the product info that one use for it is to maintain correct headset tension if you need to regularly loosen your stem to turn your handlebar sideways for storage or transport. That hadn't occurred to me.
EDIT: it's cheaper from Triton Cycles BTW
Sadly no. I have 10mm of steerer above the headset. Think I’m going to try one of these and see: https://www.sjscycles.co.uk/headsets/problem-solvers-locking-headset-spacer-for-1-18-inch-black/